Thermostats having electronics, such as programmable thermostats, may rely on an independent power source, such as a disposable battery. However, a disposable battery eventually needs to be replaced by the user. Attempts have been made to reduce power usage when the battery's remaining power is running low. For example, U.S. Pat. No. 6,513,723 discusses an HVAC system comprising a main thermostat unit and a plurality of remote sensors, each remote sensor being powered by a replaceable battery. Each remote sensor includes a user interface comprising an LCD display that displays a temperature, and further includes a temperature-up control button and a temperature-down control button. For each remote unit, when its battery level is at or below 30 percent of remaining power, the LCD display stops displaying the temperature and only displays a low-battery indication, unless one of the control buttons is pressed. If one of the control buttons is pressed, the LCD temperature display is activated for 120 seconds and then turns off again.
U.S. Pat. No. 7,537,171 also discusses an HVAC system comprising a main thermostat unit and a plurality of remote sensors, each remote sensor being powered by a replaceable battery. To extend battery life while at the same time promoting reliable communication, each remote unit is capable of transmitting signals at each of a lower power level (lower data transfer rate) and a higher power level (higher data transfer rate), and is configured to transmit signals more often at the lower power level than at the higher power level.
U.S. Patent Application Publication No. 20100084482 A1 discusses a microprocessor-controlled, wirelessly communicating programmable thermostat that is powered by household AC current. The use of household AC current is indicated as being advantageous in that “the thermostat can perform functions requiring increased power, such as processor intensive functions and wireless communications, which would not be realistic using battery power or power stealing techniques.”
Electronic thermostats can also be powered directly from an HVAC system transformer such as using a 24 VAC “common” wire (“C wire”) from the transformer, but only if one is available. When provided, the C wire has the particular purpose of supplying power for an electronic thermostat. However, many HVAC installations do not have a C-wire provided to the thermostat. For such cases, many electronic thermostats have been designed to extract power from the transformer from the circuit used to turn on and off the HVAC function, which is called “power stealing”, or “power sharing.” The thermostat “steals,” “shares” or “harvests” its power during the “OFF” periods of the heating or cooling system by allowing a small amount of current to flow through it into the load coil below its response threshold (even at maximum transformer output voltage). During the “ON” periods of the heating or cooling system the thermostat draws power by allowing a small voltage drop across itself. Hopefully, the voltage drop will not cause the load coil to dropout below its response threshold (even at minimum transformer output voltage). Examples of thermostats with power stealing capability include the Honeywell T8600, Honeywell T8400C, and the Emerson Model 1F97-0671.
Co-pending U.S. patent application Ser. No. 13/034,678 filed on Feb. 24, 2011 discusses the use of a rechargeable battery that is recharged using power harvesting techniques. However, even with the use of a rechargeable battery, there may be conditions in which power usage significantly exceeds the rate at which the battery can be recharged. Additionally there may be times when the power is temporarily interrupted such that power harvesting, or power from any source, cannot be used to recharge the battery.